1. Field of the Invention
This invention relates to a radiation image readout apparatus for reading out a radiation image stored on a recording material composed of a stimulable phosphor, such as a stimulable phosphor sheet, by exposing the recording material to stimulating rays which cause it to emit light in proportion to the stored radiation energy, and photoelectrically detecting the light emitted by the recording material. This invention particularly relates to a radiation image read-out apparatus wherein the efficiency of utilizing the stimulating rays is improved by use of a multi-layer optical filter.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to a radiation passing through an object such as the human body to have a radiation image of the object stored thereon, and is then exposed to stimulating rays which cause the stimulable phosphor sheet to emit light in proportion to the stored radiation energy. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal by a photodetector, and the radiation image of the object is reproduced as a visible image by use of the image signal on a recording medium such as a photographic film, a display device such as a cathode ray tube (CRT), or the like.
The radiation image recording and reproducing system using a stimulable phosphor sheet is advantageous over conventional radiography using a silver halide photographic material in that the image can be recorded over a very wide range (latitude) of radiation exposure. More specifically, since the amount of light emitted upon stimulation after the radiation energy is stored on the stimulable phosphor sheet varies over a wide range in proportion to the amount of said stored energy, it is possible to obtain an image having desirable density regardless of the amount of exposure of the stimulable phosphor sheet to the radiation, by reading out the emitted light with an appropriate read-out gain and converting it into an electric signal to reproduce a visible image on a recording medium or a display device.
In the aforesaid radiation image recording and reproducing system, read out of the light emitted by the stimulable phosphor sheet is conducted by one of the two methods as described below. In a first read-out method, division into picture elements is effected by the scanning with stimulating rays, and the light emitted by the stimulable phosphor sheet is detected by a light receiving device having a wide light receiving face, for example, a photomultiplier. The first read-out method is realized by a radiation image read-out apparatus wherein the stimulable phosphor sheet carrying a radiation image stored thereon is two-dimensionally scanned with stimulating rays, and the light emitted by the scanned portion of the stimulable phosphor sheet is photoelectrically detected by use of a photodetector. On the other hand, in a second read-out method, division into picture elements is effected by a light receiving device such as a two-dimensional solid state image pickup device or a semiconductor line sensor, and image signals are sequentially generated by an electric circuit. The second read-out method is realized by a radiation image read-out apparatus comprising a stimulating ray source for emitting stimulating rays to the stimulable phosphor sheet carrying a radiation image stored thereon, and a photodetector constituted by a photoelectric conversion device divided into picture elements and disposed to face the stimulable phosphor sheet.
On the other hand, in the aforesaid radiation image recording and reproducing system, the stimulable phosphor sheet is used to temporarily store the radiation image in order to reproduce the final visible image therefrom on a final recording medium. For economical reasons, therefore, it is desirable that the stimulable phosphor sheet be used repeatedly.
Further, in a mobile X-ray diagnostic station such as a traveling X-ray diagnostic station in the form of a vehicle like a bus which is provided with a radiation image recording and read-out apparatus for use in the aforesaid radiation image recording and reproducing system and moves from place to place to record radiation images for mass medical examinations, it is disadvantageous to load the mobile X-ray diagnostic station with a large number of stimulable phosphor sheets, and the number of the stimulable phosphor sheets which can be loaded on the mobile X-ray diagnostic station is limited. Therefore, it is desired to load the mobile X-ray diagnostic station with stimulable phosphor sheets which can be used repeatedly, once store the radiation images of the objects respectively on the stimulable phosphor sheets, transfer the electric image signals read out from the stimulable phosphor sheets to a recording medium having a large storage capacity, such as a magnetic tape, and circulate and reuse the stimulable phosphor sheets for further image recording and read-out operations, thereby to obtain the radiation image signals of many objects. Further, when image recording is conducted continuously by circulating and reusing the stimulable phosphor sheets, it becomes possible to increase the image recording speed in mass medical examination. This is very advantageous in practical use.
In order to reuse stimulable phosphor sheets as described above, the radiation energy remaining on the stimulable phosphor sheet after it is scanned with stimulating rays to read out the radiation image stored thereon should be erased by exposure to light or heat as described, for example, in U.S. Pat. No. 4,400,619 or Japanese Unexamined Patent Publication No. 56(1981)-12599. The stimulable phosphor sheet should then be used again for radiation image recording.
From the aforesaid viewpoint, the applicant proposed two types of the radiation image read-out apparatuses (i.e. radiation image recording and read-out apparatuses) as described below which comprise, built in a single unit, an image recording section for storing a radiation image of an object on a recording material composed of a stimulable phosphor, such as a stimulable phosphor sheet, an image read-out section for exposing the recording material to stimulating rays and photoelectrically reading out light emitted by the recording material in proportion to the stored radiation energy to obtain an image signal, and an erasing section for releasing residual radiation energy from the recording material before image recording is conducted on the recording material subjected to image read-out, whereby the recording material is circulated and reused in the apparatus.
One of the aforesaid two types of the radiation image recording and read-out apparatuses comprises:
(i) a circulation and conveyance means for conveying at least one stimulable phosphor sheet for recording a radiation image thereon along a predetermined circulation path,
(ii) an image recording section disposed on said circulation path for recording a radiation image on said stimulable phosphor sheet by exposing said stimulable phosphor sheet to a radiation carrying image information,
(iii) an image read-out section disposed on said circulation path and provided with a stimulating ray source for emitting stimulating rays for scanning said stimulable phosphor sheet carrying said radiation image stored thereon at said image recording section, and a photoelectric readout means for detecting light emitted by said stimulable phosphor sheet scanned by said stimulating rays to obtain an electric image signal, and
(iv) an erasing section disposed on said circulation path for, prior to the next image recording on said stimulable phosphor sheet for which the image read out has been conducted at said image read-out section, having said stimulable phosphor sheet release the radiation energy remaining on said stimulable phosphor sheet. This type of the radiation image recording and read-out apparatus is disclosed in Japanese Unexamined Patent Publication No. 59(1984)-192240, and will hereinbelow be referred to as the built-in type apparatus I.
The other of the aforesaid two types of the radiation image recording and read-out apparatuses comprises:
(a) a supporting material,
(b) at least one recording material fixed on said supporting material and comprised of a stimulable phosphor layer capable of storing a radiation image,
(c) an image recording section for exposing said recording material to a radiation passing through an object to have a radiation image of the object stored on said recording material,
(d) an image read-out section provided with a stimulating ray source for emitting stimulating rays for scanning said recording material carrying said radiation image stored thereon, and a photoelectric read-out means for obtaining an electric image signal by reading out light emitted by said recording material scanned and stimulated with the stimulating rays,
(e) a means for circulating said recording material on said supporting material with respect to said image read-out section for enabling reuse of said recording material by repeatedly moving said supporting material and said image read-out section with respect to each other, and
(f) an erasing means for eliminating the radiation energy remaining on said recording material prior to image recording on said recording material after the radiation image is read out therefrom at said image read-out section. This type of the radiation image recording and read-out apparatus is disclosed in Japanese Unexamined Patent Publication No. 58(1983)-200269, and will hereinbelow be referred to as the built-in type apparatus II.
On the other hand, in the aforesaid radiation image read-out apparatus, the drawback that the efficiency of utilization of stimulating rays is low has heretofore been found to arise. Specifically, since a part of stimulating rays impinging upon a surface of the recording material composed of a stimulable phosphor, such as the stimulable phosphor sheet, passes through the recording material without stimulating the stimulable phosphor and emanates from the other surface of the recording material, stimulating rays are not efficiently utilized for stimulation of the stimulable phosphor. Therefore, particularly in the case where image read-out is to be conducted with a high sensitivity, a stimulating ray source having a large output becomes necessary, and the power requirement becomes large.